This invention concerns rotary index tables and more particularly, precision rotary indexing fixtures utilized to carry out precision machining of work pieces, such as are used with numerically controlled machine tools. Such rotary index tables are very commonly employed in machining work pieces in order to carry out boring, grinding, drilling, and other operations on a work piece successively positioned at various angular positions with respect to the machining spindle. As noted, the precise position of the work piece must be carefully controlled during the indexing in order to maintain accuracy of the machining process being performed on the work piece.
It has heretofore been known and employed to use a rotary indexing table of a type using a Hirth coupling, comprising mating rings formed with serrations, one fixed to indexing top plate and the other to a stationary base structure. In operation, during indexing, the top plate is lifted clear of the serrations, indexed to the next indexed position, where it is caused to be lowered, such that the Hirth coupling teeth are again in meshing relationship. This securely locks the top plate in the indexed position.
This arrangement has been highly advantageous due to the precision afforded in controlling the angular position of the table in any indexed position. This precision arises due to the averaging effect of the serrations, in which the machining inaccuracies of the serration teeth are averaged out over the entire ring series, to thus provide a precision which is much greater than that with which the individual gear teeth can be manufactured. Another characteristic of the Hirth coupling is that the accuracy tends to improve with wear, the teeth becoming more and more uniform after repeated indexing cycles. These indexing tables were originally mounted for indexing rotation about a vertical axis and the weight of the top plate utilized to hold the mating serrations in engagement. While suitable for relatively low tool cutting reaction forces, for significant machining forces, the serrations must be held in engagement with a locking force acting to increase the gravity induced engagement pressure between the mating serration teeth.
A typical arrangement of such locking indexing tables included an outer serration ring affixed to the top plate and a second ring typically located close to the periphery of the top plate and adapted to be placed in meshing relationship. The actuator, either mechanical, hydraulic, or pneumatic, is generally mounted centrally of the indexing axis, and commonly a central member is affixed to the center of the work table and caused to be elevated and/or a locking force applied thereto, usually hydraulically. This forces the serration teeth into tighter locking engagement in order to greatly increase the rigidity of holding the top plate in position during machining operations. The outward radial location of the serrations were employed since this maximized the stability of the support afforded by the Hirth coupling. That is, the ability of the coupling to resist forces acting on the top plate tending to tip or otherwise misposition the top plate was more easily resisted as the tooth location was moved further out from the axis of rotation of the table top plate.
On the other hand, the need to apply a locking force via the central hub, resulted in the application of considerable distorting stresses acting on the top plate due to the radial distance between the point of application of the force and the reaction forces acting through the Hirth coupling serrations. Such distorting force acts to move the work piece out of precise location and reduces the rigidity of the top plate in absorbing the forces generated during machining of the work piece.
Another problem is encountered with hydraulically operated indexing tables which are utilized when relatively high levels of locking forces are required. The locking arrangement heretofore utilized is that of a double acting hydraulic cylinder which holds the top plate in locking engagement with a ring of serrations fixed with respect to a base structure. In the event of power failure or hydraulic system failure, a loss of hydraulic pressure in the hold down cylinder will allow loosening of the work piece, in turn resulting in either scrappage of the work piece due to machining inaccuracies or a radical shift of the work piece causing damage to the machine tool and/or the index table itself.
Accordingly, it is an object of the present invention to provide a rotary precision index table of the type described including axially mating serrations carried by rings mounted respectively to the top plate and to a base structure which is located close to the extreme radial outermost location on the radius of the index table in which distortions of the top plate by applied locking forces are minimized.
It is another object of the present invention in which the operation is fail-safe to insure locking of the top plate even with a failure of pneumatic or hydraulic pressure.